Concerns for environmental protection have led designers of aviation turbo engines to search for means to reduce the proportion of pollutants in the exhaust gases of the engines. It is known that the principal problems in the matter of pollution of aviation turbo engines are, on the one hand, the emission of carbon monoxide, of hydrocarbons, and of various unburnt residues during operation on the ground and, on the other hand, the emission of nitrogen oxides and of particles during take-off and during cruising at altitude.
Conventional combustion chambers are generally of optimized rating for take-off or near take-off operation. This signifies that, in the primary zone of the combustion chamber, a fraction of the air flow of the compressor is introduced so that, with the injected fuel, the fuel-air mixture in this zone would be essentially stoichiometric in these modes. Under these conditions, due to the levels of temperature and high pressures, as complete as possible a combustion is obtained, combustion yields greater than 0.99 are attained, the speeds of the chemical reaction being optimum for these stoichimoetric mixtures.
In contrast, at low ratings, at idle or nearly so, the total richness in the chamber is only about half that at take-off; in addition, the pressures and temperatures at the outlet of the compressor are lower; the result is that the chamber, with the partial charge is very much maladjusted and that the slow speed combustion efficiency rarely goes beyond 0.93. The combustion is, therefore, very incomplete, which means much higher concentrations of carbon monoxide and unburnt residues at the exhaust than under normal operation. The proportions of the pollutants are all the higher, the lower the total yield of the combustion.
However, it appears to be possible to improve the performance of a combustion chamber by acting on four factors:
The timing of vaporization of the fuel,
THE TIMING OF THE AIR-FUEL MIXTURE,
THE TIMING OF THE FRESH GAS/BURNT GAS MIXTURE,
THE TIMING OF THE CHEMICAL REACTION.
The first two times can be considered negligible at high ratings because of the pressures which are attained, but it is not so at low ratings. In fact, in order to increase the speed of the vaporization of the fuel, it must be transformed into fine droplets, which, in normal operation, is easily realized by the conventional mechanical atomizing injector, but the performance which is obtained in the lower ratings is poor. This is due to the fact that, if the fuel is well divided into droplets, these are poorly mixed with air in the primary zone and local zones would appear which have a richness which is too high. In the end, it would be necessary that each droplet would have around it the quantity of gas necessary for its vaporization and for its combustion, i.e., a quantity of gas which results in a stoichiometric mixture with the oxygen molecules after complete varporization. In order to accomplish this, systems such as aerodynamic injection have been proposed. Aerodynamic type injectors generally comprise whirling, or swirler vanes through which the air from the compressor is introduced, which serves to atomize the fuel. An air/fuel pre-mixture is thus obtained. The detailed operation and the realization of a certain number of injectors of this type are described in French Pat. Nos. 72.25840, 72.40166 and the application for the certificate of addition to the latter 73.38253, in the name of the applicant.
Another solution which can also be used is pre-atomization in which an air/fuel mixture is injected against the flow through the intermediate part of a T-shaped can. These systems are described, for example, in the French Pat. No. 71.05753 and in the French patent application No. 72.22811 in the name of the applicant.
The fresh gas/burnt gas mixture must also be advantageous because it contributes to the increase in the temperature of the carburized mixture and, therefore, aids in its atomization and consequently permits an improvement in the speed of the chemical reaction. In conventionally allowing this contact of the carburized mixture with the high temperature gas from the combustion it is desirable to arrange for a recirculation of the latter by searching for a convenient turbulence level.
All of these solutions, which allow an improvement in the combustion yield have, however, a maximum efficiency only for values sufficient for the pressures and temperatures of the air at the chamber inlet.
As far as the reaction time is concerned, it is necessary to additionally reserach an optimization of the richness of the mixture, the ideal would be to be able to obtain a stoichiometric air/fuel proportion in the flame stabilization zone, regardless of the operation of the engine. One such approach is, for example, the double chamber described in the French patent application No. 73.-8819 in the name of the applicant; this chamber is comprised of two annular concentric combustion zones, of which one is adapted for slow operation. However, this type of chamber has a considerable technological complexity, which increases its manufacturing cost, its installation is delicate, and furthermore, its weight is greater. See also U.S. Pat. No. 3,088,279.